Hardness Testing

Question 1

Why is it possible to get hardness readings +/- 3 points apart on the same metal with the same scale?

Answer 1

• Material is non-homogenous
• Human error in minor load
• Testing too close to another indent

Question 2

According to the wall charts, what is the tensile strength of the hardened steel you tested (closest value)?

Answer 2

Hardness can be relative to the ultimate tensile strength for these types of steels. The values seen on the wall chart were found experimentally to give a good idea of how a material might perform in a tensile test.

Question 3

How do your readings compare for the annealed steel using HRA and HRC scales? For hardened steel?

Question 4

How do your readings compare for the aluminum using the HR45N and the HR45T scales? Were both scales appropriate? Why or why not?

Answer 4

• 45N is N?A while 45T gave X values
• 45N is not appropriate for this material because…
• The only difference between the tests was that the Diamond cone indenter goes too far into the material surface to give a valid result when compared to the ball indenter

Question 5

How far apart must two adjacent indentations be? Why?

Answer 5

• 3X the diameter of the penetrator
• Previous tests impart stress work on the testing surface which can increase the hardness of the next test

Question 6

How far must an indentation be from the edge of the specimen? Why?

Answer 6

• 2.5X the diameter of the penetrator
• If an indentation is too close to the edge then there may be not enough material to constrain the deformation zone surrounding the indentation made. This can result in a lower hardness than the actual value.

Question 7

What is done differently when testing cylindrical surfaces/round bar?

Answer 7

• V-groove Anvil
• Correction Table

Question 8

When should the superficial scale be used?

Answer 8

Thin or soft materials

Question 9

Which scale is recommended for testing copper and annealed brass?

Answer 9

Either HRF or HRB are valid
(Just realize that there is a guide to use when selecting a scale for any given material. Just because we used HRA/HRC in lab, doesn’t mean that is the appropriate scale for everything)

Question 10

Major Loads

Answer 10

60, 100, and 150kg

Question 11

Minor Loads

Answer 11

10kg for regular scale
3kg for superficial scale

Question 12

Indentors

Answer 12

Brale/Diamond Cone
1/16” ball
1/8” ball

Question 13

When is a scale not appropriate?

Answer 13

A scale is not appropriate if the dial rotates all the way past zero, this is shown on the digital New Beuhler hardness tester as a negative number. If you get a N/A reading, consult the ASTM standards for the correct scale to use with that material.

Question 14

Major & Minor Load for HRA

Question 15

Major & Minor Load for HRC

Question 16

Major & Minor Load for HR45N

Question 17

Major & Minor Load for HR45T

Question 18

HRA Indenter

Question 19

HRC Indenter

Question 20

HR45N Indenter

Question 21

HR45T Indenter

Question 22

How does Rockwell calculate its hardness value?

Answer 22

The Rockwell testing machine calculates its hardness based on how far the indenter goes into the material. (Depth of penetration)

Based on internal calculations, the machine takes the distance and uses a scale factor to give the value.

Question 23

Hardness

Answer 23

The ability of a material to resist plastic deformation.